Lam Cheuk-ting, Ted Hui and at least 14 others have been detained over protests last year, triggered by the government's decision to pass an extradition bill allowing suspects to be sent to mainland China to stand trial.
The pro-democracy media mogul Lai and activist Chow are the highest profile arrests made under Beijing’s sweeping national security law.
Lai owns the group which publishes anti-Beijing paper Apple Daily. His arrest marks the first time the law is being used against Hong Kong's news media.
Local media said six activists were wanted on suspicion of secession or colluding with foreign forces, crimes that the new law punishes with up to life in prison.
Disqualified candidates included activist Joshua Wong, some members of the Civic Party and others who won an unofficial primary vote held by the opposition camp earlier this month.
Beijing says French politicians were being misled by a slanderous American, Australian and British campaign while threatening to invalidate British National Overseas passports held by Hong Kong residents.
President Trump orders end to Hong Kong's special status under US law to punish China for what he called "oppressive actions" against former British colony.
More than 200,000 voters took part in the election to select candidates who stand the best chance of success in Legislative Council elections in September.
Hong Kong leader Carrie Lam hailed the opening as "a historic moment" and said China's intelligence apparatus would be an "important partner" in helping to safeguard national security.
TikTok to exit Hong Kong market within days, while other technology companies, including Facebook, WhatsApp, Telegram, Google, Zoom and Twitter, suspend processing government requests for user data in the region over Beijing's new security laws.
Last week, China enacted a security law outlawing four national security crimes: subversion, secession, terrorism and colluding with foreign forces.
Hong Kong public libraries have placed certain books under review to see whether they "violate the stipulations" of a new national security law that China imposed on the financial hub.
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